This application claims the priority of German application No. 100 100 68.6-45, filed Mar. 2, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a multifuel fuel cell system and to a method for its operation.
Owing to their method of operation, fuel cells have better energy efficiency than conventional internal combustion engines, for which reason they are increasingly being used for electricity generation in both stationary and mobile applications.
Since fuel cells are normally operated with hydrogen, which can be stored only with great complexity, attempts are increasingly being made to store the hydrogen in the form of liquid fuels. Such fuels are either pure hydrocarbons or alcohols. The prior art for mobile applications, in particular in the motor vehicle field, at the moment predominantly uses methanol, which is split in a gas production unit or hydrogen reformation system into hydrogen and carbon dioxide. In practice, a complete fuel cell system comprises at least one fuel cell with coolant connection, an air supply, and a gas production unit.
Typical gas production units have a fuel tank, in particular a methanol tank; a water tank; metering pumps for methanol and water; an evaporator and a superheater for methanol and water; a reformer unit; means for carbon monoxide removal by selective oxidation, methanization or application to a membrane (membrane unit); and a burner unit for producing heat for the vaporization and reformation.
For mobile applications, the lack of a methanol infrastructure and the low storage density of methanol in comparison to fuels based on mineral oils have been found to be major disadvantages. Furthermore, the high energy vehicle efficiency of a methanol fuel cell system is virtually balanced out by the upstream chain for methanol production. Hydrogen production based on conventional liquid fuels, for example diesel, petrol or LPG, is thus increasingly being considered for mobile fuel applications. These so-called multifuel fuel cell systems normally have at least one fuel tank; a water tank; metering systems for the respective fuel or a number of different fuels and water; an evaporator and superheater for the fuel or fuels and water; a high-temperature reformer for carrying out partial oxidation (POX reformer) with shift units; means for carbon monoxide removal by selective oxidation, methanization or application to a membrane; and a burner unit for producing heat for the vaporization and reformation.
The chemical process for obtaining hydrogen from hydrocarbons is generally partial oxidation reformation in accordance with the following equation:
xe2x80x94(CH2)xe2x80x94+xc2xdO2(air)xe2x86x92H2+CO.
Another method comprises vapour reformation of hydrocarbons based on the following equation:
xe2x80x94(CH2)xe2x80x94+2H2Oxe2x86x923H2+CO2.
Combinations of the two stated processes are likewise possible, and lead to autothermal methods of operation.
The energy required to obtain hydrogen (vaporization and subsequent reformation) in such multifuel gas production systems is produced in a catalytic burner and/or during the selective carbon monoxide oxidation and/or in the shift stages.
A method for operation of a steam reformation system is known from EP 0 924 161 A2. This document relates to a system and a method for operating the system for steam reformation of a hydrocarbon. The system includes a reactor which is suitable for both partial oxidation operation and reformation operation; an evaporator; and a hydrogen separation stage and a catalytic burner device. In the system described, a first part of the catalytic burner device is in thermal contact with the reformation reactor, and a second part of the catalytic burner device is in thermal contact with the evaporator. In addition, means are provided for switching the reactor between partial oxidation operation and reformation operation, and these means include an air/hydrocarbon intermediate feedline for the reactor and a pressure-maintaining valve. According to the method, when the system is started from cold, a heating process is carried out during which the reactor is initially operated at low temperature in the partial oxidation mode and is then switched to the reformation mode, with the pressure at the same time being increased to the normal operating pressure.
A method for operation of a system for steam reformation of a hydrocarbon is known from WO 99/31012. In this case, once the system has been warmed up, the original substance to be reformed is subjected to steam reformation in the reformation reactor. When the system is started from cold, at least a part of the reformation reactor, as a multifunctional reactor unit, is operated as a catalytic burner unit in a first operating phase, with a fuel and a gas containing oxygen being supplied, and is operated as a partial oxidation unit, for partial oxidation of the original substance, in a subsequent, second operating phase. The method is distinguished by the fact that, shortly before the change from the first operating phase to the second, water is metered into the supplied mixture of fuel and gas containing oxygen. During the first operating phase, the fuel flow rate is increased as the temperature of the multifunctional reactor unit rises, and the flow rate of the gas containing oxygen is actually set to be sub-stoichiometric during the first operating phase.
The object of the present invention is to provide a multifuel fuel cell system which can be operated in a simple manner, and a method for its operation, by which the cold-starting behavior of a vehicle powered by fuel cells also achieves the desired characteristics.
According to the present invention, a starting behavior which has a minimal warming-up time for the system components can now be achieved for vehicles powered by fuel cells. In addition, it is possible substantially to prevent any undesirable cold-starting emissions, since the catalytic converter elements produce their full performance even shortly after starting.
It is preferable for the components which are provided between a first catalytic converter element and at least one fuel cell to comprise a high-temperature heat exchanger, at least one shift stage, a selective oxidizer, a catalytic burner, a high-pressure compressor and/or an evaporator. Expedient interconnection of these components makes it possible to produce a hydrogen gas of a desired purity. Furthermore, the physical separation according to the present invention between these components and the fuel cell during the first operating mode makes it possible to achieve advantageous cold-starting behavior.
The separation of and the connection between the hydrogen production unit and the at least one fuel cell are expediently provided by mechanical separating means, in particular a three-way cock. By opening and closing a three-way cock, it is possible to switch between a first operating mode and a second operating mode in a simple manner.
The at least one shift stage, the selective oxidizer and/or the catalytic burner in each case has air metered to them in an advantageous manner, in particular during the first operating mode. This makes it possible for the air to be catalytically converted with the synthetic gas or reformate with a large amount of heat being developed, by which these components are actively heated. For example, a downstream evaporator can then be heated by the hot exhaust gas from the catalytic burner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.